1. Field of the Invention
The present invention relates to the field of high density interconnect structures for interconnecting electronic components, and more particularly, to such structures employing more than one layer of conductors.
2. Background Information
A high density interconnect (HDI) structure or system which has been developed by General Electric Company offers many advantages in the compact assembly of electronic systems. For example, an electronic system such as a micro computer which incorporates 30-50 chips can be fully assembled and interconnected on a single substrate which is 2 inch long by 2 inch wide by 0.050 inch thick. Even more important, this interconnect structure can be disassembled for repair or replacement of a faulty component and then reassembled without significant risk to the good components incorporated within the system. This is particularly important where as many as 50 chips having a cost of as much as $2,000.00, each, may be incorporated in a single system on one substrate. This repairability is a substantial advance over prior connection systems in which reworking the system to replace damaged components was either impossible or involved substantial risk to the good components.
Briefly, in this high density interconnect structure, a ceramic substrate such as alumina which may be 100 mils thick and of appropriate size and strength for the overall system, is provided. This size is typically less than 2 inches square, but may be made larger or smaller. Once the position of the various chips has been specified, individual cavities or one large cavity having appropriate depth at the intended locations of differing chips, is prepared. This may be done by starting with a bare substrate having a uniform thickness and the desired size. Conventional, ultrasonic or laser milling may be used to form the cavities in which the various chips and other components will be positioned. For many systems where it is desired to place chips nearly edge-to-edge, a single large cavity is satisfactory. That large cavity may typically have a uniform depth where the semiconductor chips have a substantially uniform thickness. Where a particularly thick or a particularly thin component will be placed, the cavity bottom may be made respectively deeper or shallower to place the upper surface of the corresponding component in substantially the same plane as the upper surface of the rest of the components and the portion of the substrate which surrounds the cavity. The bottom of the cavity is then provided with a thermoplastic adhesive layer which may preferably be polyetherimide resin available under the trade name ULTEM.RTM. 6000 from the General Electric Company. The various components then placed in their desired locations within the cavity, the entire structure is heated to about 300.degree. C. which is above the softening point of the ULTEM.RTM. polyetherimide (which is in the vicinity of 235.degree. C.) and then cooled to thermoplastically bond the individual components to the substrate. Thereafter, a polyimide film which may be Kapton.RTM. polyimide, available from E. I. du Pont de Nemours Company, which is .apprxeq.0.0005-0.003 inch (.apprxeq.12.5-75 microns) thick is pretreated to promote adhesion by reactive ion etching (RIE), the substrate and chips are then coated with ULTEM.RTM. 1000 polyetherimide resin or another thermoplastic and the Kapton film is laminated across the top of the chips, any other components and the substrate with the ULTEM.RTM. resin serving as a thermoplastic adhesive to hold the Kapton.RTM. in place. Thereafter, via holes are provided (preferably by laser drilling) in the Kapton.RTM. and ULTEM.RTM. layers in alignment with the contact pads on the electronic components to which it is desired to make contact. A metallization layer which is deposited over the Kapton.RTM. layer extends into the via holes and makes electrical contact to the contact pads disposed thereunder. This metallization layer may be patterned to form individual conductors during the process of depositing it or may be deposited as a continuous layer and then patterned using photoresist and etching. The photoresist is preferably exposed using a laser to provide an accurately aligned conductor pattern at the end of the process. Alternatively, exposure through a mask may be used.
Additional dielectric and metallization layers are provided as required in order to provide all of the desired electrical connections among the chips. Any misposition of the individual electronic components and their contact pads is compensated for by an adaptive laser lithography system which is the subject of some of the Patents and Applications which are listed hereinafter.
This high density interconnect structure provides many advantages. Included among these are the lightest weight and smallest volume packaging of such an electronic system presently available. A further, and possibly more significant advantage of this high density interconnect structure, is the short time required to design and fabricate a system using this high density interconnect structure. Prior art processes require the prepackaging of each semiconductor chip, the design of a multilayer circuit board to interconnect the various packaged chips, and so forth. Multilayer circuit boards are expensive and require substantial lead time for their fabrication. In contrast, the only thing which must be specially pre-fabricated for the HDI system is the substrate on which the individual semiconductor chips will be mounted. This substrate is a standard stock item, other than the requirement that the substrate have appropriate cavities therein for the placement of the semiconductor chips so that the interconnect surface of the various chips and the substrate will be in a single plane. In the HDI process, the required cavities may be formed in an already fired ceramic substrate by conventional or laser milling. This milling process is straightforward and fairly rapid with the result that once a desired configuration for the substrate has been established, a corresponding physical substrate can be made ready for the mounting of the semiconductor chips in as little as 1 day and typically 4 hours for small quantities as are suitable for research or prototype systems to confirm the design prior to quantity production.
The process of designing an interconnection pattern for interconnecting all of the chips and components of an electronic system on a single high density interconnect substrate normally takes somewhere between one week and five weeks. Once that interconnect structure has been defined, assembly of the system on the substrate may begin. First, the chips are mounted on the substrate and the overlay structure is built-up on top of the chips and substrate, one layer at a time. Typically, the entire process can be finished in one day and in the event of a high priority rush, could be completed in four hours. Consequently, this high density interconnect structure not only results in a substantially lighter weight and more compact package for an electronic system, but enables a prototype of the system to be fabricated and tested in a much shorter time than is required with other packaging techniques.
This high density interconnect structure, methods of fabricating it and tools for fabricating it are disclosed in U.S. Pat. No. 4,783,695, entitled "Multichip Integrated Circuit Packaging Configuration and Method" by C. W. Eichelberger, et al.; U.S. Pat. No. 4,835,704, entitled "Adaptive Lithography System to Provide High Density Interconnect" by C. W. Eichelberger, et al.; U.S. Pat. No. 4,714,516, entitled "Method to Produce Via Holes in Polymer Dielectrics for Multiple Electronic Circuit Chip Packaging" by C. W. Eichelberger, et al.; U.S. Pat. No. 4,780,177, entitled "Excimer Laser Patterning of a Novel Resist" by R. J. Wojnarowski et al.; U.S. patent application Ser. No. 249,927, filed Sep. 27, 1989, entitled "Method and Apparatus for Removing Components Bonded to a Substrate" by R. J. Wojnarowski, et al.; U.S. patent application Ser. No. 310,149, filed Feb. 14, 1989, entitled "Laser Beam Scanning Method for Forming Via Holes in Polymer Materials" by C. W. Eichelberger, et al.; U.S. patent application Ser. No. 312,798, filed Feb. 21, 1989, entitled "High Density Interconnect Thermoplastic Die Attach Material and Solvent Die Attachment Processing" by R. J. Wojnarowski, et al.; U.S. patent application Ser. No. 283,095, filed Dec. 12, 1988, entitled "Simplified Method for Repair of High Density Interconnect Circuits" by C. W. Eichelberger, et al.; U.S. patent application Ser. No. 305,314, filed Feb. 3, 1989, entitled "Fabrication Process and Integrated Circuit Test Structure" by H. S. Cole, et al.; U.S. patent application Ser. No. 250,010, filed Sep. 27, 1988, entitled "High Density Interconnect With High Volumetric Efficiency" by C. W. Eichelberger, et al.; U.S. patent application Ser. No. 329,478, filed Mar. 28, 1989, entitled "Die Attachment Method for Use in High Density Interconnected Assemblies" by R. J. Wojnarowski, et al.; U.S. patent application Ser. No. 253,020, filed Oct. 4, 1988, entitled "Laser Interconnect Process" by H. S. Cole, et al.; U.S. patent application Ser. No. 230,654, filed Aug. 5, 1988, entitled "Method and Configuration for Testing Electronic Circuits and Integrated Circuit Chips Using a Removable Overlay Layer" by C. W. Eichelberger, et al.; U.S. patent application Ser. No. 233,965, filed Aug. 8, 1988, entitled "Direct Deposition of Metal Patterns for Use in Integrated Circuit Devices" by Y. S. Liu, et al.; U.S. patent application Ser. No. 237,638, filed Aug. 23, 1988, entitled "Method for Photopatterning Metallization Via UV Laser Ablation of the Activator" by Y. S. Liu, et al.; U.S. patent application Ser. No. 237,685, filed Aug. 25, 1988, entitled "Direct Writing of Refractory Metal Lines for Use in Integrated Circuit Devices" by Y. S. Liu, et al.; U.S. patent application Ser. No. 240,367, filed Aug. 30, 1988, entitled "Method and Apparatus for Packaging Integrated Circuit Chips Employing a Polymer Film Overlay Layer" by C. W. Eichelberger, et al.; U.S. patent application Ser. No. 342,153, filed Apr. 24, 1989, entitled "Method of Processing Siloxane-Polyimides for Electronic Packaging Applications" by H. S. Cole, et al.; U.S. patent application Ser. No. 289,944, filed Dec. 27, 1988, entitled "Selective Electrolytic Deposition on Conductive and Non-Conductive Substrates" by Y. S. Liu, et al.; U.S. patent application Ser. No. 312,536, filed Feb. 17, 1989, entitled "Method of Bonding a Thermoset Film to a Thermoplastic Material to Form a Bondable Laminate" by R. J. Wojnarowski; U.S. patent application Ser. No. 363,646, filed Jun. 8, 1989, entitled "Integrated Circuit Packaging Configuration for Rapid Customized Design and Unique Test Capability" by C. W. Eichelberger, et al.; U.S. patent application Ser. No. 07/459,844, filed Jan. 2, 1990, entitled "Area-Selective Metallization Process" by H. S. Cole, et al.; U.S. patent application Ser. No. 07/457,023, filed Dec. 26, 1989, entitled "Locally Orientation Specific Routing System" by T. R. Haller, et al.; U.S. patent application Ser. No. 456,421, filed Dec. 26, 1989, entitled "Laser Ablatable Polymer Dielectrics and Methods" by H. S. Cole, et al.; U.S. patent application Ser. No. 454,546, filed Dec. 21, 1989, entitled "Hermetic High Density Interconnected Electronic System" by W. P. Kornrumpf, et al.; U.S. patent application Ser. No. 07/457,127, filed Dec. 26, 1989, entitled "Enhanced Fluorescence Polymers and Interconnect Structures Using Them" by H. S. Cole, et al.; U.S. patent application Ser. No. 454,545, filed Dec. 21, 1989, entitled "An Epoxy/Polyimide Copolymer Blend Dielectric and Layered Circuits Incorporating It" by C. W. Eichelberger, et al.. application Ser. No. 07/504,760, filed Apr. 5, 1990, entitled, "A Building Block Approach to Microwave Modules", by W. P. Kornrumpf et al.; application Ser. No. 07/504,821, filed Apr. 5, 1990, entitled, "HDI Microwave Circuit Assembly", by W. P. Kornrumpf, et al.; application Ser. No. 07/504,750 filed Apr. 5, 1990, entitled, "An Ultrasonic Array With a High Density of Electrical Connections", by L. S. Smith, et al.; application Ser. No. 07/504,803, filed Apr. 5, 1990, entitled, "Microwave Component Test Method and Apparatus", by W. P. Kornrumpf, et al.; application Ser. No. 07/504,753, filed Apr. 5, 1990, entitled, "A Compact High Density Interconnected Microwave System", by W. P. Kornrumpf; application Ser. No. 07/504,769, filed Apr. 5, 1990, entitled, "A Flexible High Density Interconnect Structure and Flexibly Interconnected System" by C. W. Eichelberger, et al.; application Ser. No. 07/504,751, filed Apr. 5, 1990, entitled, "Compact, Thermally Efficient Focal Plane Array and Testing and Repair Thereof", by W. P. Kornrumpf, et al.; application Ser. No. 07/504,749, filed Apr. 5, 1990, entitled, "High Density Interconnect Structure with Top Mounted Components", by R. J. Wojnarowski, et al.; application Ser. No. 07/504,770, filed Apr. 5, 1990, entitled, "A High Density Interconnect Structure Including a Chamber", by R. J. Wojnarowski, et al.; and application Ser. No. 07/504,748, filed Apr. 5, 1990, entitled, "Microwave Component Having Tailored Operating Characteristics and Method of Tailoring" by W. P. Kornrumpf, et al. Each of these Patents and Patent Applications is incorporated herein by reference.
Any additional dielectric layers which are required for isolation between the first metallization layer and any subsequent metallization layers or for prevention of short circuits due to contact with external conductors are formed by spinning on or spraying on a solvent solution of a desired thermoplastic dielectric material. The structure is then baked to drive off the solvent in order to leave a solvent-free dielectric layer. Alternatively, in accordance with U.S. patent application Ser. No. 454,545, entitled, "An Epoxy/Polyimide Copolymer Blend Dielectric and Layered Circuits Incorporating It" a siloxane-polyimide/epoxy blend may be spun-on, dried and cured to provide this dielectric layer. Thereafter, via holes are formed as needed and a patterned metallization layer is formed thereover which is disposed in ohmic contact with underlying metal in the via holes. If needed, further dielectric and metallization layers are formed thereover in a similar manner. Unfortunately, there are a limited number of dielectric materials which are suitable for use as these upper dielectric layers because of all of the material properties they must exhibit. Not only must the dielectric material be available as a spin-on or spray-on solution, it must also provide good adhesion to the underlying dielectric and metallization and to the material of any overlying metallization or dielectric layer which may subsequently be formed thereon and it should be inherently laser ablatable or it should be rendered laser ablatable in accordance with U.S. patent application Ser. No. 456,421, entitled, "Laser Ablatable Polymer Dielectrics and Methods".
By a thermoplastic polymer material, we mean a polymer material which after multiple cycles of heating and cooling substantially retains its initial melting point or glass transition temperature. That is, no substantial cross-linking of the material takes place during the heating, melting and resolidification process. Such polymers are suitable as adhesive layers for bonding higher temperature materials, including polymers to substrates and may also themselves be used as layers to be bonded to substrates through the use of lower temperature adhesive layers. The glass transition temperature of a polymer is the temperature above which the viscosity of the polymer decreases greatly, thereby allowing the polymer to flow and also to bond to other materials. When cooled below this glass transition temperature, the thermoplastic polymer "resolidifies" and remains adherent to objects with which it is in intimate contact. By a thermoset polymer material, we mean a polymer material in which cross-linking takes place the first time it is heated or during its preparation process, such that the material either no longer melts or melts at a much higher temperature than it did prior to its initial heating or its formation, as the case may be.
While the use of spin-on or spray-on dielectric layers for the second and higher dielectric layers of a high density interconnect structure is effective, it has a number of potential drawbacks and process complications. In particular, such dielectric layers must be baked to drive off their solvent. The baking time and temperature profiles involved can adversely affect some electronic components. Further, in some situations, there is poor adhesion between adjacent dielectric layers. In other situations, excessive stress in the dielectric layers or at the interface between adjacent dielectric layers can adversely affect the quality and reliability of a high density interconnect structure. During the coating of further dielectric layers, the solvent vehicle tends to redissolve the surface portion of an already formed, unreacted, thermoplastic dielectric layer on which it is disposed. While this tends to improve adhesion, it can also result in excessive interfacial stress and cracking or crazing of the dielectric layers which renders the structure unusable.
There are known techniques for using thermoset materials as adhesives in the formation of multilayer printed circuit boards. These include spinning on a precursor of a thermoset material and reacting it in place to form a thermoset dielectric layer. Once such a layer has reacted, it is no longer soluble in the solvent which is used in forming the next layer with the result that damage to underlying layers does not occur during the formation of subsequent layers. Such materials are available from Sheldahl Corporation.
Another thermoset technique is the type of laminate used in making laminated circuit boards. These include the copper dielectric laminate available under the trademark Pyrolux.RTM. from Dupont. These systems use acrylate adhesives which become thermoset at a curing temperature of about 135.degree. C. Unfortunately, acrylate adhesives are not considered sufficiently thermally stable for use in most high density interconnect structures of the type to which the present invention is directed and once reacted generally don't solvent at all. In particular, it is considered desirable to be able to operate these high density interconnect structure systems in the vicinity of 200.degree. C. or higher. Acrylates are not considered usable above 150.degree. C. Epoxies also exhibit thermal instabilities which prevent their use at these temperatures. In particular, most epoxies turn tan at 150.degree. C. and turn black at 180.degree. C.
Printed circuit boards made by either of these techniques cannot be disassembled for repair and most of those printed circuit boards which are faulty must be discarded. Printed circuit boards of whatever type are fully tested prior to mounting expensive components thereon. Consequently, expensive chips are not committed to faulty printed circuit boards. While this lack of repairability is a disadvantage, it is more than offset for printed circuit board applications by the combination of relatively high testing yield and the high durability the thermoset structure imparts to the circuit board.
While use of a thermoset structure is beneficial in the printed circuit board art, the use of such thermoset systems is unacceptable in a high density interconnect structure of the type to which this invention is directed because the expensive chips are put in place before the interconnection structure is built. As a result, any fault in a thermoset high density interconnect structure would require scrapping not only of the interconnection structure itself, but all of the chips as well. Thus, the conversion of a high density interconnect structure of the type described above into one having a thermoset structure is not considered a solution to the problems associated with the formation of multilayered high density interconnect structures.
The use of multiple thermoset dielectric layers which are laminated to the structure using a thermoplastic adhesive to provide such a multilayer structure in which each dielectric layer includes a thermoset upper sublayer and a thermoplastic lower sublayer as discussed in U.S. Pat. No. 4,783,695 has not been implemented because of the tendency for the early laminated layers to shift, deform or otherwise change during the lamination of subsequent layers in a way which breaks via connections between layers. Consequently, an alternative process for providing additional dielectric layers is desirable.
A further problem which we have found in packaging completed, fully tested high density interconnect structures is a development of inter-layer short circuits during the packaging process in which force is applied to the top of the high density interconnect structure in order to provide an adequate bond between the package base and the high density interconnect structure substrate. We have determined that these short circuits are a result of the soft or pliable nature of the spun-on dielectric layers used for the second and subsequent dielectric layers in those structures which results in shifting of the dielectric and the creation of short circuits. This means that the circuits are not as rugged as desired and also means that thickness of the dielectric layers is not stable at least in the vicinity of some of the conductor runs or particular conductor patterns. This raises an additional concern for microwave applications of this structure because of the need in such structures to provide a known, fixed dielectric layer thickness in order to provide transmission lines with their intended impedances in order that the structure will exhibit its intended microwave performance. Thus, there is a need for an improved process and high density interconnect structure configuration which eliminates or minimizes these problems.
This problem should be at least partially solved by the multiple lamination process taught in related application Ser. No. 07/546,936, entitled, "Multiple Lamination High Density Interconnect Process and Structure Employing Thermoplastic Adhesives having Sequentially Decreasing Tg's", since that process provides a thermoset or high temperature thermoplastic laminated layer over each metallization layer. Those high temperature layers should solve the problem of short circuits. However, that structure does not eliminate the problem of shifting and/or changing thicknesses of the thermoplastic portions of that structure. Further, the use of multiple T.sub.g adhesive layer could potentially aggravate the problem of thermoplastic shift because of the use of successively lower T.sub.g materials in multilayer structures.
Consequently, there is a need for a further improvement in such high density interconnect structures which retains their repairability while improving their durability.